B = baseline, E1-E7 = sleep-level testing days, and R1-R3 = recovery days with further testing to track return toward baseline. In the graph above you can see a decline in both the five-hour- and three-hour-a-night groups, with limited decline from baseline values in the seven-hour-a-night group, and no average decline in the nine-hour-a-night group. Image courtesy of Belenky G, westensen NJ, Thorne DR, Thomas ML, Sing HC, Redmond DP, Russo MB, & Balkin TJ. Patterns of performance degradation and restoration during sleep restriction and subsequent recovery: a sleep dose-response study. Wiley online library. 2003. http://doi.org/10.1046/j.1365-2869.2003.00337.x .